This invention relates to a yarn brake having a support, a bearing pin, two brake elements mounted on the bearing pin, of which at least one is axially movable on the bearing pin, and a bias mechanism holding the two brake elements in abutment.
Yarn brakes of this kind are inclined in a high degree to contamination, since spin finishes or residues of oil, paraffin or the like adhering to the yarn easily collect on the yarn-contacting or brake surfaces. It is therefore already known (DE 3 504 739 A1) to arrange the brake elements rotatably on the bearing pin. The rotatable mounting of the brake elements serves the purpose of utilizing the tangential forces exerted on the brake elements by the running yarn to make the brake elements shift with a rotary movement and thereby effect constant self-cleaning of the brake elements. However, when using conventional brake elements pressed together by compression springs, the desired free rotation of the brake elements is so strongly impeded by the frictional forces exerted by the compression springs, that particles of dirt which have collected do not then automatically emerge from the space between the brake elements even when large enough openings are provided in their walls. The same applies in principle to yarn brakes in which the brake elements are pressed together with the aid of permanent magnets, of which one is fixed in position (DE 195 31 579 C1), because a corresponding braking moment is exerted on the brake elements by the magnet arrangements. In spite of the tendency towards self-cleaning present in principle, the yam brake therefore has to be cleaned at relatively short intervals of time, which with a machine processing or using yarn with a plurality of yarn guides, e.g. a 96 system circular knitting machine, necessitates expensive cleaning work with corresponding machine down time.
In addition there is the danger that brake elements which are not rotating or are barely rotating wear out quickly or at least grooves and sharp edges result thereon, which must be avoided to save the yarn.
Accordingly yarn brakes have already become known whose brake elements are either forced to rotate with the aid of drive means, which are coupled to the brake elements in the manner of slipping clutches (DE 2 758 334 C2) or are so coupled to a device creating oscillations (EP 0 499 218 A1) that the effect of a force promoting the rotational movement of the brake elements results. Such auxiliary devices are however not acceptable from the economic point of view in machines which use or process a large number of yarns, because they require a huge constructional outlay.
Finally a yarn brake of the kind initially specified is known (DE 4 301 507 C2) in which the brake elements have central holes with diameters which are substantially greater than the outer diameter of the bearing bodies, so that an off-centre mounting relative to their axis of rotation results. However, such a design of the yarn brake makes the use of pre-tensioning element necessary which consist exclusively of magnetic inserts attached to the brake elements. In consequence a self-cleaning action which can be achieved under some circumstances is faced with the disadvantage that the bias on the brake elements and thus the braking force exerted on the yarn can only be adjusted by changing the magnetic inserts, which involves a large loss of time and can only take place in comparatively coarse steps, while continuous adjustment of the braking force is made possible with the springs provided in conventional yam brakes.